NAMD 2.6b1 includes a new option vdwGeometricSigma that provides the proper Lennard-Jones parameter combining rule for the OPLS force field.

The parameters are apparently available via a link from http://mackerell.umaryland.edu/charmm_ff.shtml

"Topology and parameter files for the AMBER, OPLS and BMS force fields are now included, allowing for modeling studies with CHARMM using these force fields. See the non_charmm subdirectory."

Another useful link is: http://norma.mbg.duth.gr/index.php?id=research:howto:using_the_opls-aa_forcefield

NOTE: (quoting from the README file of the above mentioned tar): "The topology and parameter files here include only the protein portion of the OPLS-AA force field. User-defined topology and parameter files are required for other functionalities."